Composition for molds for metal casting



aiented Jan. 28, 1941 D UNITED STATES PATENT OFFICE 2,229,946comrosrrron Fon Moons non METAL oas'rmo George Clyde Van Allen,Maplewood, N. 1., assignor to Baker & Compa corporation of New Je ny,Inc'., Newark, N. J., a my No Drawing. Application February 8, 1938,-

1 Serial No. .189,319

1 Claim.

This invention relates to investment compositions for the making ofrefractory molds for use in casting metals, alloys and other suitablecastillg materials. The composition is particularly intended for use inmolds for casting dental inlays, dentures and the like, although thecomnet, for example calcined gypsum or plaster of Paris. Otheringredients or agents for the purpose of accelerating or retardinghardening, setting or expansion of the composition, or to modify theplasticityof the composition to facilitate mixing and shaping thereof,are also frequently included in such investment compositions.

The customary procedure in making molds of such compositions and castingarticles therein,

consists in shaping a preliminary wax model or pattern of the object tobe cast, for examplea *dental inlay, mixing the investment compositionwith a suitable liquid, for example water, constructing the mold aroundthe pattern, allowing the mold to set, heating the set mold slowly todrive out the moisture and melt and burn out the wax pattern, andcontinuing heating until the mold is suillciently hot to compensate forthe shrinkage of the cast material as well as to eliminate all wax andresidue from same so as to leave a cavity corresponding in shape andsize to the model, then pouring the molten casting material, for examplegold, into the heated mold cavity and allowing the casting to cool.

Generally the molds are heated to a temperatureof from 900 F. to 1500 F.before the casting is formed, which causes the mold to expand, and

. the casting material after having been put in the mold cavity usuallyshrinks during cooling. Therefore, in order to obtain a casting whichshall conform exactly in size and shape to the model, the expansion ofthe mold must correspond as exactly as possible'to the shrinkage of thecasting material. Generally this expansion is slightly above 1%, forexample in the case of casting gold, about 1.25%.

While such investment compositions comprising only aground siliceousrefractory material, such as silica, and a binder such as plaster ofParis, have the property oi expansion under heat, the degree ofexpansion is not sufilcient to compensate entirely for the shrinkage ofthe casting 5 material. As an example, we find that a compo-- sitioncomprising seventy percent of silica and thirty percent (30%) of plasterof Paris has a maximum expansion of about eighty-six hundredths of onepercent (0.860%) at a temperm ature of fifteen hundred (1500) degrees F.which is insuflicient to compensate for the average shrinkage of thecasting material of from one (1) to one and one-quarter (1%) percent. Adecrease of the silica content and a corresponding increase in theplaster of Paris content will result in a considerably smaller expansionof the composition during heating.

To increase the expansion of the investment composition, it is customarypractice to include therein so-called expansion agents or expansion 20promoters, such as oxalicacid or citric acid or sodium chloride.

Prime object of my invention is to provide an investment compositionwhich shall include a new and improved combination of constituents or 25ingredients one or more of which shall make it possible to obtainexactly the desired degree of expansion of the composition duringheating thereof. 30

I have discovered that the desired expansion characteristics ofinvestment compositions can be obtained, and the degree of expansion ofsuch compositions can be increased substantially, by incorporating insuch compositions small quan-- titles of one or more substances of thegroup consisting of ammonium chloride, aluminum chloride, ammoniumbromide, and sodium nitrate.

As examples, sodium nitrate and ammonium bromide together may be addedas expansion agents to the other constituents of the composition, orammonium chloride and aluminum chloride together may be used asexpansion agents;

and if desired all four of my enumerated expansion agents may be usedtogether.

The addition of one or more of my expansion agents to an investmentcomposition remarkably improves the expansion characteristics ofcomposition and strikingly increases the degree of expansion. Whereas,as above pointed out a so Expansion agent 55 53 Sodium nitrate .7 1.035Ammonium chloridm. 1. 040 Aluminum chloride. 1. 180 Ammonium bromidei 1. 205

The proportions of my expansion agents in the investment composition maybe varied from one-quarter of one percent (0.25%) to about three percent(3%) by weight. The preferred range lies between three-quarters of onepercent (0.75%) to two percent (2%), although the most satisfactoryproportions with the proportions of the other ingredients now used, arefrom one percent (1%) to one and three-tenths percent (1.30%).

The proportion of the refractory base or filler, for example silica, maybe varied widely, for example from forty percent to eighty-five percent(85%), while the proportion of the binder, for example calcined gypsumor Portland cement, may also vary within a wide range, for example fromfifteen percent (15%) to sixty percent Preferably, in all cases, thematerials comprise particles of different sizes.

Each of my expansion agents may be used alone, or in combination with.others of my expansion agents, or in combination with other expansionagents or promoters.

Also, my expansion agents may be utilized in investment compositionswith ingredients or agents for accelerating or retarding Setting orhardening of the compositions, or for modifying plasticity or otherqualities or characteristics of the compositions.

Having thus described my invention, what I claim is:

An investment composition comprising from forty percent (40%) to eightypercent of silica, from fifteen percent (15%) to sixty percent (60%) ofplaster of Paris, and from one-quarter percent (0.25%) to three percent(3%) of ammonium bromide.

G. CLYDE VAN ALLEN.

